1. Field of the Invention
The subject invention is directed generally to materials for lining dentures and, more particularly, to improved permanent soft denture liners based on new organosilicon polymers and monomers. According to the present invention, proper selection of an organosilicon polymer and a siloxane monomer as its crosslinking agent, and by optimizing the liner formulation, lining materials can be produced which have enhanced resiliency, good bonding to the denture base, and minimum chemical degradation and water sorption.
2. Description of the Related Art
With respect to the invention, it is believed that some discussion of the background of denture liners would be helpful. In this manner, the invention can be put in better perspective.
Denture liners are utilized to interface between the interior surfaces of dentures and the denture-bearing mucosa, or gum tissue, of the patient. Relining is a process in which a relatively thin film of polymer is added to the inside surface of the denture to obtain an improved fit with the contours of the denture-bearing mucosa. The technique includes making an impression of the denture-bearing mucosa after applying the polymer to the denture. The denture liners are of several types and are used for a variety of reasons. They are generally classified into three groups:
(1) hard relining materials; PA0 (2) tissue conditioners; and PA0 (3) soft lining materials.
The hard relining materials are generally used for adjustments or a quick-fixes of ill-fitting dentures. They can be applied to accomplish on-the-spot refitting of dentures.
To date, two types of materials have been utilized to make hard relining materials. Each type of material typically consists of two parts, a powder and a liquid. In the first type (type 1), the powder is made of polymethylmethacrylate (PMMA) polymer beads mixed with benzoyl peroxide, a free radical polymerization initiator, and the liquid is a combination of monomer methyl methacrylate (MMA) mixed with a plasticizer, and a polymerization accelerator, e.g., a tertiary amine. In the second type (type 2), the powder is made of polyethylmethacrylate (PEMA) beads mixed with benzoyl peroxide, and the liquid is a mixture of butylmethacrylate monomer (BMA), and a polymerization accelerator such as a tertiary amine.
Examples of such hard denture relining materials are found in Antonucci et al (U.S. Pat. No. 5 037 473). Although curable by visible light, the monofunctional monomers disclosed polymerize but do not crosslink; and, unfortunately, those liners possess all of the drawbacks common to hard denture repair materials as enumerated below.
In use, the two parts (either type) are mixed and applied to the interior surface of the denture. The liner is hardened slowly by polymerization, and during the term of cure, while the material is still soft, the denture is inserted and an impression of the oral tissue is recorded. The denture is then removed and the remainder of the curing takes place with the denture removed.
These hard liners have certain undesirable drawbacks, however. The presence of the plasticizer generally gives the hard reline materials a rather low Tg (glass transition temperature) and this often leads to dimensional instability in the oral environment. The monomer MMA in type 1 liners is an irritant to many patients and so can produce tissue inflammation. In addition, since the cured liner is rather hard, and the thickness of the liner is difficult to control, hard reliners are usually used only as temporary solution to the ill-fitting denture. They do not provide the long-term solution desired by denture wearers.
Tissue conditioners are soft polymeric materials used to treat mucosa irritated by other denture lining materials. They are mixed at chairside, placed in the denture, and the denture is then seated in the patient's mouth. The materials will conform to the anatomy contours of the residual ridge, gel in that position, but not harden and continue to flow slowly after application.
Tissue conditioners typically consist of polymers of higher methacrylates, such as polyethylmethacrylate, a solvent, such as ethyl alcohol, and a plasticizer, e.g., butylphthalyl butylglycolate. The solvent and the plasticizer are mixed with the polymer immediately prior to use, and the mixture is then applied to the surface of the denture. The setting of the tissue conditioning liner involves the swelling of the polymer by the solvent and the diffusion of the plasticizer into the polymer matrix causing the polymer to become soft and resilient. Tissue conditioners are initially very soft after application, having a modulus of elasticity value of approximately 0.05 MPa after one hour compared with a value of 2000 MPa for typical acrylic denture base materials. The softness of the polymer liners, however, is temporary. Loss of solvent and leaching of the plasticizer from the polymer matrix occur continuously in the oral environment and producing hardening of the liners in 2-3 days.
On a short-term basis, however, these materials are able to perform both as tissue conditioners and functional impression materials due to the viscoelastic properties of the materials. The elastic nature of the materials cushions the cyclic forces of mastication; and the viscous properties of the materials allow excellent adaptation to the irritated denture-bearing mucosa.
As to the third category of denture liners, both temporary and permanent soft lining materials are currently being used. Temporary soft liners are chemically similar to tissue conditioners except that they are formulated to retain their softness for a longer period of time. They may also be used as tissue conditioners and as diagnostic aids to ascertain whether the patient would benefit from permanent soft liners.
The denture-bearing mucosa of patients vary, and some are more sensitive or thinner than others and cannot tolerate the masticatory force applied to the hard denture base. For these patients, a permanent soft liner is needed on the surface of the hard denture as the cushion.
The temporary soft liners mainly consist of polymers of higher methacrylates, e.g., polyethylmethacrylate, a solvent, e.g., ethyl alcohol, and a plasticizer, e.g. butylphthalyl butylglycolate. These and others are disclosed in Graham et al, J. Prosthetic Dentistry, Volume 2, No. 4, pp. 422-8 (1989). As with the tissue conditioners, the solvent and plasticizer are mixed with the polymer immediately prior to use, and the mixture is then applied to the surface of the denture. Swelling of the polymer by the solvent, and diffusion of the plasticizer into the polymer matrix cause the polymer to become soft and resilient. As with the tissue conditioners, loss of solvent and leaching of plasticizer occur continuously in the oral environment and eventually cause the hardening of the liners. A number of permanent soft denture liners are commercially available and some others are currently under development. The majority of commercial soft liners are based on silicone polymers, the so-called polysiloxanes of the general structure (I) ##STR1## where n is an integer having a value of from 1 to about 10,000, and where R and R' are organic radicals such as methyl or phenyl groups or hydrogen atoms.
Two kind of polysiloxanes are commonly used for soft liners. They are vinyl-terminated polydimethylsiloxane (II), and hydroxyl-terminated polydimethylsiloxane (III), ##STR2## where n is an integer having a value from 1 to about 5,000. ##STR3## where n is an integer having a value from 1 to about 5,000.
For ease of application, silica fillers are usually added to the polysiloxanes so that the polymer-filler mixtures have the consistency of pastes. The filler particles also enhance the mechanical properties of the elastomers after cure.
The vinyl-terminated siloxane polymer mixes (II) are heat cured and contain an amount of benzoyl peroxide which acts as a free-radical polymerization and crosslinking initiator. Upon application of heat, the initiator promotes the crosslinking of the polymer molecules and thus converts the linear polymer into a highly resilient elastomer. The heat activated crosslinking reaction is in the nature of a pure addition reaction which does not produce any by-product. The liners are often referred to as heat curing silicone soft liners.
The curing of hydroxyl-terminated polydimethylsiloxanes, on the other hand, is in the nature of a condensation reaction. Although it requires no heat, the reaction requires a crosslinking agent, e.g., tetraethyl silicate, and a catalyst, such as dibutyl tin dilaurate. Upon the mixing of the polysiloxane with the crosslinking agent and the catalyst, the catalyst initiates the crosslinking of the polysiloxane molecules at room temperature and converts the linear polymer into a highly resilient elastomer. The condensation reaction produces ethyl alcohol as the by-product. The liners are often called cold curing silicone soft liners.
Desired characteristics for permanent soft liners include permanent resiliency, high dimensional stability, adequate adhesion to the denture base polymer, PMMA, adequate wettability in the oral environment, compatibility with oral tissue, i.e., non-toxic, non-irritant, and such materials should be incapable of sustaining bacterial growth.
Although a number of polysiloxane-based permanent soft liners are currently available, actual reports on their clinical performance have been meager. Based on the chemical structure of the materials, the nature of the crosslinking reactions which occur during cure, and available reports on the physical, chemical and mechanical properties of the liners; however, it appears clear that current polysiloxane-based soft liners have many drawbacks. None of the presently available materials comes close to fulfilling all the above requirements.
The heat cured type of soft liners, for example, are made from vinyl-terminated polydimethylsiloxane which is highly hydrophobic, and thus has a very low wettability. Additionally, the vinyl-terminated polydimethylsiloxane is chemically dissimilar to the more polar PMMA denture base polymer material, and consequently, its adhesion to the PMMA denture base is poor.
The cold cured hydroxyl-terminated polydimethylsiloxane soft liners rely on an organo tin compound as the catalyst to crosslink the polymers. The residual organo tin catalyst is often an irritant to the patients. In addition, the siloxane bonds formed by the condensation reactions are susceptible to hydrolysis. The cold cured polysiloxane soft liners have further been reported to have poor wettability, poor dimensional stability, and poor rupture resistance.
Organopolysiloxanes with diverse uses have been proposed. Examples include tapes and release paper adhesives using mixtures of certain methacrylate ester-modified organopolysiloxanes disclosed in Wewers et al (U.S. Pat. No. 5,034,491). In Weitemeyer et al (U.S. Pat. No. 4 678 846), such material is used as a metal adhesive, and by Lien et al (U.S. Pat. No. 4 528 081) for electronic potting applications. Nothing is disclosed in these references, however, with regard to possible usefulness or compatibility of these compounds in an oral environment. Likewise, Jacobine et al (U.S. Pat. No. 4,952,711) disclose yet additional polysiloxane or silicone materials having unsaturated terminal groups which may also be useful as adhesives or for potting. They disclose a bis((meth)acryloxy)propenyl terminated polydimethylsiloxane as one possible preferred unsaturated acrylic functional silicone prepolymer which, when mixed with a silicone prepolymer having a plurality of organothiol groups and a thiolene cure catalyst, is curable to a solid crosslinked polyorganosiloxane. The possible reactivity of these unsaturated terminal groups, however, is an additional unknown with respect to judging compatibility of the material in an oral environment.
Dental resin compositions for use as crown or bridge resins or filling resins in dentistry which are made of modified siloxane copolymers are disclosed by Yamazaki et al in U.S. Pat. No. 4,826,893. It is further known to cure certain silicone polymers with UV light as shown in Preiner et al (U.S. Pat. No. 4,595,471) and Eckberg (U.S. Pat. No. 4,988,743).
Soft denture liners based on materials other than polysiloxanes are also known. The hydrophilic polymer poly(hydroxyethyl methacrylate) which turns into a hydrogel upon exposure to water has been reported to be the main ingredient of a commercial soft liner called Hydron.TM. (trademark of National Patent Development Corporation of New York, N.Y.). The material, upon exposure to water, absorbs significant amount of water (up to 20 weight %), and thus is soft, compliant, and resilient in the oral environment. Unfortunately, because of significant water solubility, the dimensional stability and mechanical properties such as rupture resistance of the material are poor.
Other polymeric materials have been used in the oral environment. These include poly(ethylenically unsaturated) carbamoyl isocyanurates used as restoratives, prostheses, and sealants as disclosed by Mitra in U.S. Pat. No. 4 648 843. Other known restorative materials include those described by Heid et al (U.S. Pat. No. 5 028 638). They disclose a dental composite filling material containing a polymerizable (meth)acrylic compound in which camphorquinone is employed as a photoactivator with p-diaklylaminobenzene sulphonamide. Other methacrylate materials which are polymerizable or crosslinkable by ultraviolet or visible light are disclosed in Waller (U.S. Pat. No. 4,746,686).
Experimental soft denture liner materials based on polyphosphazene fluoroelastomer (PNF) have been disclosed by Gettleman et al in U.S. Pat. No. 4 543 379. These are high polymers containing an inorganic backbone of alternating nitrogen and phosphorus atoms. Using ethylene glycol dimethacrylate as the crosslinking agent, BaSO.sub.4 as a filler, and lauroyl peroxide as a thermal initiator, the firmness of the crosslinked PNF can be varied from soft to firm by varying the amount of filler and the crosslinking agent. The PNF-based liners have been reported to bond well to PMMA.